This invention relates to a starter with a planet gear speed reducer, and more particularly to an improvement of a mechanism for receiving the impact stress which may occur in the power transmission mechanism of the starter (hereinafter referred to as "an impact stress receiving mechanism", when applicable).
In a conventional starter with a planet gear speed reducer, the mechanism for receiving the impact stress which may occur in the power transmission mechanism thereof has been disclosed by Japanese Utility Model Application Examined Publication No. 45081/1987. The conventional starter, as shown in FIG. 3, has a ring gear 4 which has its inner cylindrical wall formed into an internal gear 2 which is engaged with planet gears 1, and its outer cylindrical wall formed into a helical spline gear 3. The ring gear 4 is inserted into the spigot joint 6 of the front frame 5 in such a manner that the helical spline gear 3 of the ring gear 4 is engaged with the helical spline gear formed in the inner cylindrical wall of the spigot joint of the front frame 5. An annular rubber member 7 is interposed between the ring gear 4 and an auxiliary plate 8, so as to elastically receive the axial thrust of the ring gear 4 which is moved forwardly (to the right in FIG. 3) along the helical spline gear by the reaction against the motion of the planet gears 1 inscribed in the ring gear 4.
In other words, in the conventional starter, the reaction force against the rotary motion of the planet gears 1 is elastically received by the annular rubber member 7 through the screw motion of the ring gear 4, and the impact which may occur in the power system thereof is smoothly received.
However, the conventional starter described above is disadvantageous in that it is rather difficult to manufacture the ring gear 4 by cold forging because it has the spur gear formed in the inner cylindrical wall and the helical spline gear formed in the outer cylindrical wall. The front frame having the helical spline gear formed in the spigot joint is generally made of die cast aluminum, and therefore, in the case where the helical spline gear is formed in the inner cylindrical wall of the front frame, the pattern drawing operation is rather difficult, and the resultant product is low in accuracy.
Accordingly, both the ring gear and the front frame must be formed by machining; that is, they cannot be produced on a large scale.